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Selection of germanium lithium drift detectors for gamma ray spectroscopy
NUCLEAR INSTRUMENTS AND METHODS
54 (19 6 7) 147-149; ©
NORTH-HOLLAND PUBLISHING CO.
SELECTION OF GERMANIUM LITHIUM DRIFT DETECTORS FOR GAMMA RAY SPECTROSCOPY* J. M. McKENZIE and P. F. DONOVAN
Bell Telephone Laboratories, Inc., Murray Hill, New Jersey, U.S.A. and A. C. GYNN Rutgers, The State University, New Brunswick, New Jersey, U.S.A. Received 12 June 1967 A few very high resolution large germanium lithium drift detectors have been reported « 4 keV 60CO, > 20 cm 3). The resolution of the large detectors currently available, however, is much poorer than these exceptional units. Smaller detectors with
high resolution are much more easily obtainable. Thus in most gamma ray spectroscopy it is necessary to compromise between the size and the resolution of the available detectors.
Germanium lithium drift gamma ray detectors are now widely used to resolve complex gamma ray spectra 1). Many nuclear physics experiments involve looking for a weak gamma line in a high gamma background. In these experiments the spectral information that can be obtained from a flux of gamma rays, in a given time, is dependent both on the size and the resolution of the available detectors. A very few high resolution large germanium lithium drift detectors have been reported [18 cm 3, 3.2 key2) and 44 cm 3, 3.8 keY 3) for the 1.33 MeY 60Co gamma line]. The resolution of the vast majority of large detectors currently available, however, is much poorer than these exceptional units. Smaller detectors with high resolution are much more easily obtainable. Thus in most cases involving gamma ray spectroscopy it is necessary to compromise between the size and the resolution of the available detectors. The large detectors have a higher ratio of the counts in the photopeak to total counts. The increase in size of the detector, however, is accompanied by deterioration of the resolution. This arises both from the greater number of imperfections in the larger crystals and to a lesser extent from the added capacitance loading the input of the preamplifier. The amount of degradation expected from the latter is shown in fig. 1. This is the resolution, as a function of capacitance, of a preamplifier using 2 SF5868 FET in parallel as the input stage. The FET's are operated at room temperature. The design of the preamplifier is fairly conventional [for example, ref. 4)]. We simulated a weak gamma line in a high gamma background by exposing the detector to a mixed source Consisting of approximately 0.3 mCi 60CO and 40 }lCi
137 Cs. We then recorded the spectra, fig. 2, in equal times and in the same flux for three sizes of detector. The resolutions are those for both amplifier and detector: I. A coaxial detector 20 em 3 volume, 7 keY resolution; 2. A planar detector 5 cm 3 volume, 3.9 keY resolution; 3. A planar detector 2 cm 3 volume, 2.8 keY resolution. In each case the detector resolves the 137CS line at 661 keY but for the smaller detectors the line is more pronounced than for the larger detector. A useful
• Work in part supported by the National Science Foundation.
8
6
o
20
40 60 80 ADDED CAPACITANCE (prJ
100
Fig. I. The resolution of a preamplifier, with 2 SFS868 FET's in parallel as the input stage, as a function of added capacitance. The unit was operated at room temperature. A 60CO source and germanium detector were used for the calibration.
147
4000
148
i
1
DET.27 COAXIAL'" 20 .. CAPACITANCE 74.F IES. C..• 7 ..V PULSEI 4.2 hV
2000 1
u
:
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100
200
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300
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400
500 0
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100
300
400
500
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1000 DET.37 PLANAI"" 2 •• CAPACITANCE 14pF
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u
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'...."":-:-:','"',..",--=:±I::----::~L~",,-l 200
300
400
500
CHANNEL NO.
Fig. 2. The gamma ray spectrum of a mixed source of 60CO and 137CS obtained, in equal times, with three sizes of germanium det The detectors were exposed to the same gamma flux. The smaller detector shows the better resolving power. DEl. 27 COAXIAL"" 20 .. CAPACITANCE 74 ~F
5200· DET.36 PLANAR ow 5 .. CAMCITANCI 24~F .. S. C.- 3.9 ktN
1ft
~
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u
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RIS. C... 2.11 bY PULlIR 2. 25ktN
......
~.
500
CHANNEL NO. Fig. 3. The gamma ray spectrum of a mixed source of 60CO and 137CS obtained at the same count rate and for equal times with sizes of detector. This demonstrates more clearly the better resolving power of the smaller detector.
SELECTION OF GERMANIUM LITHIUM DRIFT DETECTORS
figure of merit is the ratio of the peak height of the 137CS line above the 60Co background to the 60Co background. This figure is 0.2 for the 20 cm 3 detector, 0.35 for the 5 cm 3 and 0.39 for the 2 cm 3. Thus more spectral information can be obtained per unit time from the smaller rather than the larger detector. In many nuclear physics experiments counting rate in the detector or associated electronics is a limit rather than beam current. We have simulated increase in beam current in the smaller detectors by moving the source closer to the smaller detectors. The count rate was thus adjusted to that observed in the larger detector. Fig. 3 shows the spectra of the three detectors taken for equal times but at the same counting rate.
149
The statistics in the smaller detectors has now improved and their usefulness is much more apparent. Thus where spectral information is desired a smaller high resolution detector can provide more information per unit time than a larger detector of poorer resolution.
References 1) J. M. Wyckoff, IEEE Trans. Nucl. Sci. NS-14, no. 1(1967) 634.
2) F. Cappellani, W. Fumagalli, M. Henuset and G. Restelli,
Nucl. Instr. and Meth. 47 (1967) 121. 3) Private communication, 1. L. Fowler, AECL, Chalk River,
Ontario, Canada. 4) K. F. Smith and J. E. Kline, IEEE Trans. Nucl. Sci. NS-13,
no. 3 (1966) 477.
54 (19 6 7) 147-149; ©
NORTH-HOLLAND PUBLISHING CO.
SELECTION OF GERMANIUM LITHIUM DRIFT DETECTORS FOR GAMMA RAY SPECTROSCOPY* J. M. McKENZIE and P. F. DONOVAN
Bell Telephone Laboratories, Inc., Murray Hill, New Jersey, U.S.A. and A. C. GYNN Rutgers, The State University, New Brunswick, New Jersey, U.S.A. Received 12 June 1967 A few very high resolution large germanium lithium drift detectors have been reported « 4 keV 60CO, > 20 cm 3). The resolution of the large detectors currently available, however, is much poorer than these exceptional units. Smaller detectors with
high resolution are much more easily obtainable. Thus in most gamma ray spectroscopy it is necessary to compromise between the size and the resolution of the available detectors.
Germanium lithium drift gamma ray detectors are now widely used to resolve complex gamma ray spectra 1). Many nuclear physics experiments involve looking for a weak gamma line in a high gamma background. In these experiments the spectral information that can be obtained from a flux of gamma rays, in a given time, is dependent both on the size and the resolution of the available detectors. A very few high resolution large germanium lithium drift detectors have been reported [18 cm 3, 3.2 key2) and 44 cm 3, 3.8 keY 3) for the 1.33 MeY 60Co gamma line]. The resolution of the vast majority of large detectors currently available, however, is much poorer than these exceptional units. Smaller detectors with high resolution are much more easily obtainable. Thus in most cases involving gamma ray spectroscopy it is necessary to compromise between the size and the resolution of the available detectors. The large detectors have a higher ratio of the counts in the photopeak to total counts. The increase in size of the detector, however, is accompanied by deterioration of the resolution. This arises both from the greater number of imperfections in the larger crystals and to a lesser extent from the added capacitance loading the input of the preamplifier. The amount of degradation expected from the latter is shown in fig. 1. This is the resolution, as a function of capacitance, of a preamplifier using 2 SF5868 FET in parallel as the input stage. The FET's are operated at room temperature. The design of the preamplifier is fairly conventional [for example, ref. 4)]. We simulated a weak gamma line in a high gamma background by exposing the detector to a mixed source Consisting of approximately 0.3 mCi 60CO and 40 }lCi
137 Cs. We then recorded the spectra, fig. 2, in equal times and in the same flux for three sizes of detector. The resolutions are those for both amplifier and detector: I. A coaxial detector 20 em 3 volume, 7 keY resolution; 2. A planar detector 5 cm 3 volume, 3.9 keY resolution; 3. A planar detector 2 cm 3 volume, 2.8 keY resolution. In each case the detector resolves the 137CS line at 661 keY but for the smaller detectors the line is more pronounced than for the larger detector. A useful
• Work in part supported by the National Science Foundation.
8
6
o
20
40 60 80 ADDED CAPACITANCE (prJ
100
Fig. I. The resolution of a preamplifier, with 2 SFS868 FET's in parallel as the input stage, as a function of added capacitance. The unit was operated at room temperature. A 60CO source and germanium detector were used for the calibration.
147
4000
148
i
1
DET.27 COAXIAL'" 20 .. CAPACITANCE 74.F IES. C..• 7 ..V PULSEI 4.2 hV
2000 1
u
:
-.
o
100
200
:
300
-- ..-.......,- ---
I
400
500 0
CHANNEl NO.
!
100
2000
...'"
g
DEl. 36
PLANAI'" 5 .. CAPACITANCE 24,F IES. Co·, 3.9 koV
1000-
u
".
"".-."
....... " I
o
200
300
----~
Soo
400
0
'--__~:__.....,~1. __ . __.1
200
100
300
400
500
CHANNEL NO.
1000 DET.37 PLANAI"" 2 •• CAPACITANCE 14pF
io'" 500
IU. C.6O 2.11 keV PULSEI 2.2hoV
u
¥;",,~.,,~~··L ...~_••_:....."'.:..-,".._~..v,;· ,.:;;....... , ,,"•. _.:"
L
j
o ······100····200"· "300
.... -.:... ~. ".
I
.........,.~.~:
.","~"".""'~-
I I
'400500
0'100'
'...."":-:-:','"',..",--=:±I::----::~L~",,-l 200
300
400
500
CHANNEL NO.
Fig. 2. The gamma ray spectrum of a mixed source of 60CO and 137CS obtained, in equal times, with three sizes of germanium det The detectors were exposed to the same gamma flux. The smaller detector shows the better resolving power. DEl. 27 COAXIAL"" 20 .. CAPACITANCE 74 ~F
5200· DET.36 PLANAR ow 5 .. CAMCITANCI 24~F .. S. C.- 3.9 ktN
1ft
~
2600
u
_--..--,-'..., 100 5000 . DET.37 PLANAR ... 2 .. CAPACITANCI I4pF
RIS. C... 2.11 bY PULlIR 2. 25ktN
......
~.
500
CHANNEL NO. Fig. 3. The gamma ray spectrum of a mixed source of 60CO and 137CS obtained at the same count rate and for equal times with sizes of detector. This demonstrates more clearly the better resolving power of the smaller detector.
SELECTION OF GERMANIUM LITHIUM DRIFT DETECTORS
figure of merit is the ratio of the peak height of the 137CS line above the 60Co background to the 60Co background. This figure is 0.2 for the 20 cm 3 detector, 0.35 for the 5 cm 3 and 0.39 for the 2 cm 3. Thus more spectral information can be obtained per unit time from the smaller rather than the larger detector. In many nuclear physics experiments counting rate in the detector or associated electronics is a limit rather than beam current. We have simulated increase in beam current in the smaller detectors by moving the source closer to the smaller detectors. The count rate was thus adjusted to that observed in the larger detector. Fig. 3 shows the spectra of the three detectors taken for equal times but at the same counting rate.
149
The statistics in the smaller detectors has now improved and their usefulness is much more apparent. Thus where spectral information is desired a smaller high resolution detector can provide more information per unit time than a larger detector of poorer resolution.
References 1) J. M. Wyckoff, IEEE Trans. Nucl. Sci. NS-14, no. 1(1967) 634.
2) F. Cappellani, W. Fumagalli, M. Henuset and G. Restelli,
Nucl. Instr. and Meth. 47 (1967) 121. 3) Private communication, 1. L. Fowler, AECL, Chalk River,
Ontario, Canada. 4) K. F. Smith and J. E. Kline, IEEE Trans. Nucl. Sci. NS-13,
no. 3 (1966) 477.